Ventilation-Perfusion Matching

Question 1

In a healthy upright subject ventilation is distributed unevenly in a vertical manner with more ventilation going preferentially to the _____ of the lung. Intrapleural pressure is less negative or higher @ the ____ of the lung.

Answer 1

In a healthy upright subject ventilation is distributed unevenly in a vertical manner with more ventilation going preferentially to the bottom (base) of the lung (like blood flow due to gravity; furthermore, there is no reflex to redistribute the blood, that is why it stays @ the bottom). Intrapleural pressure is less negative or higher @ the base of the lung.

Distribution of ventilation is measured by inhaling a radioactive marker and measuring the activity at different levels.

This phenomenon of ventilation distribution is due to the combined effect of the following two factors:

a. A vertical gradient in pleural pressure due to the weight of the lung tissue. Pleural pressure is less negative at the base of the lung due to the effect of gravity. -8.5 @ the apex & -1.5 @ the base. Transmural pressure = Pin - Pout, so @ end expiration Pin=0 so the transmural pressure of the apex & base would be 8.5 & 1.5 mmHg, with transmural pressure being higher @ the apex of the lung.
b. The compliance property of the lung. As you recall, the static pressure-volume curve is curved such that compliance is high at small lung volume and becomes smaller at higher volumes. If you take the lung out of the chest, the compliance would be the same in all regions, and the lung would inflate evenly in which case the distribution of ventilation would be even. (Perfusion, however, would be uneven due to gravity, as discussed below. The effect of gravity in the upright posture is much greater on perfusion than on ventilation.) Therefore, in the intact condition in an upright subject, ventilation is uneven and could contribute to a mismatch between ventilation and perfusion.

In the erect individual, intrapleural pressure is more negative at the apex compared to the base. Thus, alveoli are more distended at the apex than at the base. During inspiration though, there is a greater increase in alveolar size at the base; therefore, there is more ventilation at the base of the lung.

Question 2

Compliance is ______ @ the base of the lung & _____ @ the apex of the lung; therefore, the apex of the lung is _____ inflated @ rest & the base of the lung is _____ inflated @ rest. Therefore, the apex of the lung has a ______ transmural pressure than the base since it is already filled a little bit before inspiration.

Answer 2

Compliance is higher @ the base of the lung & lower @ the apex of the lung; therefore, the apex of the lung is more inflated @ rest & the base of the lung is less inflated @ rest. Therefore, the apex of the lung has a higher transmural pressure than the base since it is already filled a little bit before inspiration.

Apex is more expanded = stiffer = less compliant = less change in volume = less ventilation.

The alveoli @ the bottom expand more since they are more collapsed & compliant. The apex is already partially inflated, so they expand less & are less compliant. Therefore, ventilation is greater @ the base of the lung.

See pg. 295

Question 3

The resistance to blood flow is _____ @ the base of the lung since the capillaries are more distended @ the base and, therefore, blood flow tends to ____ from the top to the bottom of the lung. In other words, blood flow is _____ @ the base of the lung due to the dramatic effect of gravity.

Answer 3

The resistance to blood flow is lower @ the base of the lung since the capillaries are more distended @ the base and, therefore, blood flow tends to increase from the top to the bottom of the lung. In other words, blood flow is higher @ the base of the lung due to the dramatic effect of gravity.

See pg. 296

Question 4

Furthermore, the effect of gravity on blood flow is _____ than the effect of gravity on ventilation as can be seen in the V/Q ratio (ventilation/flow). Nevertheless, the V/Q ratio is higher @ the _____ of the lung, why?

Answer 4

Furthermore, the effect of gravity on blood flow is larger than the effect of gravity on ventilation as can be seen in the V/Q ratio (ventilation/flow). Nevertheless, the V/Q ratio is higher @ the apex of the lung, why?

The ratio of ventilation to perfusion (V/Q) increases toward the apex of the lung because the flow, Q decreases more than V, ventilation, which increases the overall ratio. @ the base of the lung, blood flow increases more than ventilation increases so that the ratio is lower.

The V/Q ratio is important in determining the partial pressures gasses in the various regions in the lung.

Ventilation & perfusion is higher in the base of the lung, but it has a smaller VQ ratio.

See pg. 297

Question 5

Mixed venous blood has PO2 = ___ and PCO2 = ____ mm Hg. When blood leaves the pulmonary capillary it has PO2 and PCO2 the same as in the alveoli (____ and ___ mm Hg, respectively).

Answer 5

Mixed venous blood has PO2 = 40 and PCO2 = 45 mm Hg. When blood leaves the pulmonary capillary it has PO2 and PCO2 the same as in the alveoli (100 and 40 mm Hg, respectively).

Question 6

Describe alveolar dead space, what V/Q ratio does it have?

Answer 6

a region of the alveolar compartment in which there is ventilation but no perfusion with blood; therefore, the composition of air would approach that of inspired air @ high V/Q ratios, around 150mmHg O2 & 0 mmHg CO2.

Question 7

Describe what happens @ low V/Q ratios

Answer 7

a region of the alveolar compartment in which there is perfusion but no ventilation with blood due to blockage of the airways = abnormal right-left shunt; therefore, the composition arterial blood would approach that of venous blood @ low V/Q ratios, PO2 = 40 and PCO2 = 45 mm Hg.

See pg. 298-299

Question 8

Abnormal right to left shunt has a ___ VQ ratio.

Answer 8

LOW

A pulmonary shunt is a physiological condition which results when the alveoli of the lungs are perfused with blood as normal, but ventilation (the supply of air) fails to supply the perfused region. In other words, the ventilation/perfusion ratio (the ratio of air reaching the alveoli to blood perfusing them) is zero. A pulmonary shunt often occurs when the alveoli fill with fluid, causing parts of the lung to be unventilated although they are still perfused.

Question 9

Histamine is a broncho____, but it is also a vaso_____.

Answer 9

bronchoconstrictor (it constricts pulmonary venules)

vasodilator

Question 10

VQ scans measure the ___. The normal VQ ratio is __.

During aging that alveolar dead space ____ since there is some ventilation occurring @ a high VQ ratio. Also with increasing age, there is _____ shunt flow since there is part of a curve for an older person is over a low VQ ratio.

Answer 10

VQ scans measure the VQ ratio. The normal VQ ratio is 1.

During aging that alveolar dead space increases since there is some ventilation occurring @ a high VQ ratio. Also with increasing age, there is increasing shunt flow since there is part of a curve for an older person is over a low VQ ratio.

Therefore, VQ mismatch can occur from shunting or alveolar dead space. This is probably due to the increased compliance that occurs with aging since proteases are digesting the lung tissue. Therefore, lung structure changes as a function of age.

See pg. 301.

Question 11

Remember that normal PA = ___mmHg, Pc = ___mmHg, Pa = ____mmHg, so that normal gradient is ___mmHg.

Answer 11

Remember that normal PA = 104mmHg, Pc = 104mmHg, Pa = 100mmHg, so that normal gradient is 4mmHg.

Question 12

Memorize/ understand pgs. 303 - 307, I will need to know them for the test

Answer 12

-

Question 13

Describe the normal A-a gradient, values of PAO2, PcO2, PaO2, & why PcO2>PaO2.

Answer 13

PAO2 = 104mmHg
-alveoli

PcO2 = 104mmHg
-capillary

Since gas equilibration occurs across the alveolar-capillary membrane, in the functional alveolar compartment PAO2 must equal PcO2. If they are not equal, then the functional PAO2 = PcO2

PaO2 = 100mmHg
-arterial

Gradient = PAO2-PcO2 = 4mmHg

PaO2<PcO2 due to normal left-left shunt

Note that END-TIDAL PO2 = PAO2

Note that if the DLCO is normal, I can rule out a diffusion problem

Question 14

Describe inspiratory hypoxemia with a normal A-a gradient:

1) when it occurs
2) values of PAO2, PcO2, PaO2
3) A-a gradient =
4) will O2 therapy help?

Answer 14

1) @ high altitudes where PAO2 is decreased
2) PAO2 = 80, PcO2 = 80, PaO2 = 76
3) 4, so normal
4) Yes, give them 100mmHg to give them the O2 that they would normally get @ sea level

Question 15

Describe hypoxemia with a increased A-a gradient:

1) when it occurs
2) values of PAO2, PcO2, PaO2
3) A-a gradient =
4) will O2 therapy help?

Answer 15

1) PcO2<104
2) PAO2 = 104, PcO2 = 80, PaO2 = 76
3) 28
4) Yes, keep A-a gradient @ 4 & get PaO2 to 100 so:

76 + 24 = 100, add 24 to PAO2 so 104 + 24 = 128mmHg

Give patient enriched O2 with 128mmHg O2

Question 16

Describe hypoxemia with a increased A-a gradient & abnormal right-left shunt:

1) when it occurs
2) values of PAO2, PcO2, PaO2
3) A-a gradient =
4) will O2 therapy help?

Answer 16

1) V/Q decreases due to mixing of venous blood with arterial blood so PaO2 decreased.
2) PAO2 = 104, PcO2 = 104, PaO2 = 76
3) 28 because blood shunted away from PcO2
4) O2 therapy will not help because PAO2 is already fully saturated with blood & there are no seats on the bus left .

Question 17

Describe alveolar dead space hypoxemia with a increased A-a gradient:

1) when it occurs
2) values of PAO2, PcO2, PaO2
3) A-a gradient =
4) will O2 therapy help?

Answer 17

1) End tidal PAO2 is very high > PcO2, but functional PAO2 is decreased because V/Q increases

Note that END-TIDAL PO2 = PAO2

Total ventilation = VA + VD, if we increase VD, less VA

2) PAO2 = 80 (functional), PcO2 = 80, PaO2 = 76, Palveolar-dead-space = 150 (inspired air)
3) 4, so normal gradient
4) Yes, O2 therapy with gas enriched with oxygen helps since we can drive PAO2 up to increase PaO2 (no diffusion problems).

76 + 24 = 100, add 24 to PAO2 so 80 + 24 = 104mmHg

Give patient enriched O2 with 104mmHg O2

Question 18

Should I give O2 therapy for asthma?

Answer 18

Yes, for asthma give O2 therapy.

Question 19

A 49-year-old man has a pulmonary embolism that completely blocks blood flow to his left lung. As a result, which of the following will occur?

Answer 19

Alveolar Po2 in the left lung will be approximately equal to the Po2 in the inspired air

The ventilation/perfusion (V/Q) ratio in the left lung will be infinite because Q=0

Question 20

The partial pressures of gases in the alveolar compartment are determined by the ratio of ventilation to perfusion (V/Q). This ratio is higher in the apex of the lung compared to the base of the lung. Where the V/Q ratio is higher, the PO2 is _____ and the PCO2 is _____.

Answer 20

The partial pressures of gases in the alveolar compartment are determined by the ratio of ventilation to perfusion (V/Q). This ratio is higher in the apex of the lung compared to the base of the lung. Where the V/Q ratio is higher, the PO2 is elevated and the PCO2 is decreased (so apex has higher O2 & less CO2).

A hypoxemia with an enlarged alveolar-arterial O2 gradient A–a gradient of oxygen could be due to either a ventilation/perfusion mismatch or a diffusion limitation. However, the lung-diffusing capacity test DLCO eliminates a diffusion limitation. Right-to-left shunts also cause an increased A-a gradient, but breathing 100% O2 would not increase arterial PO2 to the levels observed here. Hypobaric conditions and hypoventilation can result in hypoxemias, but they do not change the A-a gradient of oxygen.

Question 21

When the V/Q ratio decreases, the lung unit is moving toward ____ventilation. Thus, P02 would _____ and PCO2 would _____.

Answer 21

When the V/Q ratio decreases, the lung unit is moving toward hypoventilation. Thus, P02 would decrease and PCO2 would increase.

Question 22

If a patient has a anatomic dead space of 130 ml, a physiologic dead space of 170 ml, and a minute ventilation reate of 7.2 L/min while breathing at 10 breaths per min, what is the rate of alveolar dead space ventilation?

Answer 22

0.4 L/mn

Physiologic dead space is the sum of anatomic plus alveolar dead space. Thus, alveolar dead space is 170 – 130 = 40 ml, or 0.04 L. Alveolar dead space ventilation is the respiratory rate (breaths/min) times the alveolar dead space (L/breath), or 10 x 0.04 = 0.4 L/min.

Question 23

If a patient has an anatomic dead space of 120 ml, no alveolar dead space, and a minute ventilation rate of 7.2 L/min while breathing at 10 breaths/min, what is the rate of alveolar ventilation?

Answer 23

6.0 L/min

This patient has no alveolar dead space, so the physiologic dead space equals the anatomic dead space. Alveolar ventilation is total ventilation minus physiologic dead space ventilation, or 7.2 – (10 x 0.12) = 6 L/min.

Question 24

Physiologic dead space =

Answer 24

Physiologic dead space = anatomic + alveolar dead space

Physiologic dead space is the sum of anatomic plus alveolar dead space.